Existing 3G/4G FDD (Frequency Division Duplex) terminals use a full duplex design, and a transmitting channel and a receiving channel may operate simultaneously. In a conventional radio frequency (RF) architecture, a duplexer may be an essential device and may mainly function as follows: (1) it may converge and provide separation between the transmitting channel and the receiving channel, i.e. attenuating noise of a RF signal of the transmitting channel at a receiving frequency band in order for it not to interfere with a receiving signal.
Separation between the transmitting channel and the receiving channel may be provided so that the receiving channel is required to operate under a very weak condition (currently typically being −110 dBm). However, the transmitting channel may be a high power channel which can reach an intensity of 28 dBm. Due to nonlinearity of a RF system, there may be a very strong stray out-band under the condition of a 28 dBm major wave. The stray out-band may directly feed into the receiving terminal if not separated at the receiving frequency band, and the intensity may be higher than an intensity of received useful signals, thereby finally affecting receiving performances.
A typical RF structure of a FDD mobile terminal is shown in FIG. 1. Taking a WCDMA FDD terminal as an example, we may analyze the design of its receiving system as follows.
The typical receiving sensitivity of an existing WCDMA terminal may be −110 dBm.
The power of DPDCH (Dedicated Physical Data Channel) may be −120.3 dBm. A channel coding rate for WCDMA sensitivity test may be 12.2 kbps, and a coding gain may be 10×1 g (3.84 MHz/12.2) which may be equal to 25 dB.
A decoding threshold for QPSK modulation mode for WCDAMA may be 5.2 dB. A 2 dB margin may need to be reserved, and so an input signal-to-noise ratio (SNR) of a demodulation module may be required to be 7.2 dB.
Therefore, noise at an input terminal of the demodulation module may be lower than −120.3+25−7.2=−102.5 dBm/3.84 MHz=−168.343 dBm/Hz;
In view of the noise indicator of a receiver being typically 5 dB, the noise at the input terminal of the demodulation module may be lower than −173.343 dBm/Hz. And the thermal noise of the system KBT=−200+26.022=−173.977 dBm/Hz=−108.13 dBm/3.84 MHz;K(Boltzmann constant)=1.38×10−20mJ/K,B=3.84 MHz (65.843 dB), T=290 K.
The output noise of an existing typical amplifier may be −160 dBm/Hz (output of wireless transceiver)+28 dB (typical amplification gain of amplifier at receiving frequency band)=−132 dBm/Hz=−66.16 dBm/3.84 MHz.
Therefore, a typical duplexer is required to provide at least isolation of 173.343−132 =41 dB. Due to this large provided isolation, the insertion loss (IL) of an existing duplexer is larger, and is very large especially under the condition of high frequency and when the transmitting frequency band and the receiving frequency band are near. For example, the insertion loss of a duplexer used in WCDMA BC2 is over 2.5 dB; the main reason is as follows: the transmitting frequency band may be from 850 MHz to1910 MHz, while the receiving frequency band may be from 1930 MHz to 1990 MHz, and so it may be very difficult to make a band-pass filter with only a center of 1950 MHz and a transition band of 20 MHz.
Such a large insertion loss may cause the following problems:
(1) Power consumption problem: under the condition of a large insertion loss, in order to have sufficient output power, the output power of the amplifier may need to be enhanced and accordingly the power consumption may be increased.
(2) Heat dissipation problem: increase of output power of the power amplifier and increase of the power consumption may result in increased heat. Heat of the power amplifier of the existing WCDMA terminal may be very large, which may affect battery and user's experience.
(3) Cost problem: cost of a device with high technical indices may be high, resulting in a high cost of a whole terminal.